The present invention is directed to the treatment of sour hydrocarbon distillate to remove mercaptan compounds by contacting the distillate with an alkaline solution. More particularly, this invention describes an improved apparatus and process comprising a first treatment of the sour hydrocarbon distillate in the presence of a fiber bundle and a second treatment over a bed of supported oxidation catalyst.
Processes relating to the treatment of sour distillate hydrocarbons are described in many patents. For instance U.S. Pat. Nos. 3,758,404, 3,977,829 and 3,992,156 describe mass transfer apparatus and processes involving the use of fiber bundles which are particularly suited for such processes. U.S. Pat. No. 4,675,100, which is hereby incorporated by reference herein for all purposes as if fully set forth, describes apparatus and process whereby a sour hydrocarbon distillate stream is treated in two oxidation zones within the same reaction vessel to oxidize mercaptans, comprising a first treatment in the presence of a fiber bundle and a subsequent treatment over a bed of supported oxidation catalyst. According to this process, hydrocarbon containing mercaptan compounds to be oxidized is contacted with an aqueous alkali metal hydroxide solution, an oxidizing agent, preferably air, and a soluble metal phthalocyanine catalyst while in contact with a bundle of elongated fibers contained in a conduit. Upon disengagement from the fiber bundle, the hydrocarbon and the aqueous alkali metal hydroxide solution separate by gravity in a separation zone of the vessel where the aqueous alkali metal hydroxide solution becomes a lower phase and the hydrocarbons containing the oxidized mercaptan compounds which remain as an upper oil phase. The separated hydrocarbon phase is then passed upwardly, through a packed catalyst bed, in annular arrangement about the conduit containing the bundle fibers. There, the remaining difficulty oxidizable mercaptan compounds are further oxidized in contact with a catalytic composite consisting of a metal phthalocyanine disposed on a carbon support. According to the aforementioned process fresh aqueous alkali metal hydroxide solution, also referred as caustic solution hereinafter, is fed at about the top end of the catalyst bed to flow in contact with and countercurrently with the hydrocarbon from the top end to the bottom end of the bed, in order to keep the bed causticized at all times.
However, in practice, it has been observed that this method of causticizing the bed is very ineffective and fails to adequately causticize the bed while the unit is in operation. Inability to continuously causticize the bed while in operation results in the depletion of the caustic reservoir that is initially put in the bed in a period of about one to four weeks by reacting with acidic impurities found in the hydrocarbon feeds. As a result, the process must be interrupted at frequent intervals in order to maintain the mercaptan oxidation efficiency of the catalyst bed. During these frequently required recausticization process interruptions, operators reverse the hydrocarbon flow to insure that the caustic solution flows downwardly through the carbon bed. Therefore, feeding fresh caustic at about the top end of the catalyst bed and in countercurrent flow to the hydrocarbon flowing through the bed, according to the teachings of the prior art, does not accomplish effective continuous causticization of the catalyst bed. On the other hand shutting down the process on short regular time intervals in order to treat the bed with caustic has a significant adverse effect on the economic efficiency of the overall process. In addition, unit interruptions increase the risk of introducing impurities in the process. It follows from the foregoing that an urgent need exists in developing a process that would eliminate the aforementioned problems that are associated with prior art processes.